Mechanical and structural characterisation of extrusion moulded SHCC

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MECHANICAL AND STRUCTURAL

CHARACTERISATION OF EXTRUSION MOULDED

SHCC

CHRISTO RIAAN VISSER

THESIS PRESENTED FOR THE DEGREE OF MASTER OF SCIENCE

AT THE DEPARTMENT OF CIVIL ENGINEERING OF THE

UNIVERSITY OF STELLENBOSCH

SUPERVISOR: PROF. G.P.A.G.

VAN

ZIJL

December 2007

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DECLARATION

I, the undersigned, hereby declare that the work contained in this thesis report is my own original work and that I have not previously in its entirety or in part submitted it at any university for a degree.

Signature: ________________________________

Date: ____________________________________

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SUMMARY

SHCC (Strain-Hardening Cement-based Composite) is a type of HPFRCC (High Performance Fibre Reinforced Cement-based Composite) that was designed and engineered to overcome the weaknesses of ordinary concrete. It shows a high ductility as it can resist the full tensile load at a strain of more than 3%. This superior response is achieved with multiple cracking under tensile loading which has a pseudo strain-hardening phenomenon as a result.

The purpose of the research project reported in this thesis document was to design and build a new piston-driven extruder for the production of SHCC as well as R/SHCC (reinforced SHCC) elements and to investigate and characterise the structural and mechanical behaviour of extrusion moulded SHCC.

A new piston-driven extruder, specifically for academic purposes, was designed based on the principles of fluid flow mechanics. Although fluid flow is not an ideal model to represent the flow of viscous material through an extruder, it was deemed sufficient for this specific study. A new extruder with the capacity to extrude SHCC and R/SHCC was built. Provision was made that this extruder can be fitted with extruder dies and transition zones of varying shapes and sizes.

A comparative study between unreinforced as well as reinforced cast SHCC and extruded SHCC as well as a suitable R/C (Reinforced Concrete) was conducted. Three-point bending tests, representative of the envisioned structural application, were performed on specimens of each of the composites.

The unreinforced cast SHCC and especially the unreinforced extruded SHCC have a comparative level of performance to the cast R/C. These specimens displayed a similar cracking pattern of multiple cracks, although less pronounced in the extruded SHCC. The extruded SHCC has superior first cracking and ultimate strength in comparison to cast SHCC, but with accompanying lower ductility.

The reinforced SHCC specimens failed in a combination of flexure and shear. The extruded R/SHCC specimens formed multiple diagonal cracks before failure, while the cast R/SHCC specimens formed only a few diagonal cracks, before delaminating along the reinforcement. The higher shear capacity and thus the ability to form multiple diagonal cracks of the extruded R/SHCC

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can be ascribed to the better fibre orientation of the specimens in the longitudinal direction, while the cast specimens have a random orientation of fibres. R/SHCC and especially extruded R/SHCC could be a far superior structural material to R/C.

Mechanical characterisation of extruded SHCC was done with the use of uni-axial tensile and compressive tests. The results of these tests were compared with the results of uni-axial tensile tests previously performed on cast SHCC as well as uni-axial compressive tests that were performed on cast SHCC in this research study.

The extruded SHCC displayed superior tensile performance in terms of first cracking and ultimate strength in comparison to cast SHCC, but with accompanying lower ductility. In terms of compressive performance the extruded SHCC has a higher ultimate strength, but with a lower ductility than cast SHCC. The extruded SHCC also has a much higher E-modulus than cast SHCC. This can partly be attributed to the difference between the water/binder ratios of the cast and extruded SHCC, but can mainly be ascribed to the lower porosity as a result of high extrusion forces involved in the manufacturing of extruded SHCC.

A simple bending model for SHCC has also been introduced. This model is based on the mechanical characteristics of SHCC. The model somewhat underestimates the resistance moment of the extruded and cast SHCC, but this underestimation is more pronounced in the case of the cast SHCC. Various reasons for the underestimation is discussed, but it is postulated that the main reason for the difference in experimentally determined and the calculated resistance moment of the cast SHCC is the possible variation in ingredient properties and specimen preparation and testing, since the characterisation of the cast SHCC was done over a long period of time and by different researchers. The bending model is however deemed sufficient for the design purposes of SHCC.

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iv

OPSOMMING

SHCC (“Strain-Hardening Cement-based Composite”) is ‘n tipe HPFRCC (“High Performance Fibre Reinforced Cement-based Composite”) wat ontwerp is om die swakhede van gewone beton te oorkom. Hierdie materiaal het ‘n hoë duktiliteit en kan die volle trekkrag weerstaan met ‘n vervorming van meer as 3%. Hierdie uitstaande gedrag word gekenmerk deur meerdere krake wat vorm gedurende ‘n trek belasting wat vervormingsverharding tot gevolg het.

Die doel van die navorsingsprojek wat weergegee word in hierdie tesis dokument was om ‘n nuwe suier-aangedrewe ekstrueerder vir die produksie van SHCC sowel as R/SHCC (bewapende SHCC) te ontwerp en te bou en om die strukturele en meganiese gedrag van ge-ekstrueerde SHCC te ondersoek en te karakteriseer.

‘n Nuwe suier-aangedrewe ekstrueerder, spesifiek for akademiese doeleindes, is ontwerp gebaseer op die beginsels van vloeistof vloeimeganika. Alhoewel vloeistof vloeimeganika nie ‘n ideale model is vir die voorstelling van die vloei van ‘n viskose materiaal deur ‘n ekstrueerder nie, word dit beskou as aanvaarbaar vir die doeleindes van hierdie spesifieke studie. ‘n Nuwe ekstrueerder met die kapasiteit om SHCC en R/SHCC te ekstrueer is gebou. Voorsiening is ook gemaak dat ekstrueerder vorms (“dies”) en oorgangsones van verskillende vorms en groottes aan die ekstrueerder geheg kan word.

‘n Vergelykende studie tussen onbewapende sowel as bewapende gegote en ge-ekstrueerde SHCC, sowel as ‘n gepasde R/C (“Reinforced Concrete”) is uitgevoer. Drie-punt buigtoetse, verteenwoordigend van die voorgestelde strukturele toepassings vir SHCC, is uitgevoer op proefstukke van elk van die bogenoemde materiale.

Die meganiese gedrag van die onbewapende gegote SHCC en spesifiek die onbewapende ge-ekstrueerde SHCC is vergelykbaar met die meganiese gedrag van gegote R/C. Hierdie proefstukke het ooreenstemmende kraakpatrone van veelvuldige krake getoon, alhoewel dit minder prominent was in die geval van ge-ekstrueerde SHCC. Die ge-ekstrueerde SHCC het hoër eerste kraak- en maksimum sterktes in vergelyking met gegote SHCC, maar met gepaardgaande laer duktiliteit. Die bewapende SHCC proefstukke het in ‘n kombinasie van buig en skuif gefaal. Die ge-ekstrueerde R/SHCC proefstukke het meerdere diagonale krake gevorm voor faling, terwyl die gegote R/SHCC proefstukke slegs ‘n paar diagonale krake gevorm het, voordat dit langs die

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bewapening gedelamineer het. Die hoër skuifkapasiteit van ge-ekstrueerde SHCC en dus die vermoë om meerdere diagonale krake te vorm, kan toegeskryf word aan die longitudinale orientasie van vesels van die proefstukke, terwyl gegote proefstukke se vesels meer lukraak georienteerd is. R/SHCC en spesifiek ge-ekstrueerde R/SHCC kan‘n superieure strukturele materiaal in vergelyking met R/C wees.

Die meganiese karakterisering van ge-ekstrueerde SHCC is gedoen met die gebruik van direkte trek- en druktoetse. The resultate van die hierdie toetse is vergelyk met die resultate van direkte trektoetse uit ‘n vorige studie op gegote SHCC,, sowel as met die uitslae van direkte druktoetse wat op gegote SHCC in hierdie navorsingstudie gedoen is.

Die ge-ekstrueerde SHCC het superieure trekgedrag in terme van eerste kraak en maksimum sterktes in vergelyking met gegote SHCC getoon, maar met gepaardgaande laer duktiliteit. In terme van drukgedrag het die ge-ekstrueerde SHCC ‘n hoër maksimum druksterkte, maar met ‘n laer duktiliteit in vergelyking met die gegote SHCC. Die ge-ekstrueerde SHCC het ook ‘n veel hoër E-modulus as gegote SHCC. Dit is gedeeltelik as gevolg van die verskil in die water/binder verhouding van die gegote en ge-ekstrueerde SHCC, maar kan grootliks toegeskryf word aan die laer porositeit van ge-ekstrueerde SHCC as gevolg van die hoë ekstrusie kragte.

‘n Eenvoudige buigmodel vir SHCC word ook voorgestel. Hierdie model is geabseer op die meganiese gedrag van SHCC. Die model onderskat die weerstandsmoment van ge-ekstrueerde SHCC sowel as gegote SHCC, maar hierdie onderskatting is meer prominent in die geval van gegote SHCC. Verskeie redes vir hierdie onderskatting word genoem, maar dit word beweer dat in die geval van gegote SHCC dit grootliks as gevolg van moontlike variasies in die materiaal eienskappe en proefstukke se voorbereiding en toetsing is, aangesien die karakterisering van die gegote SHCC oor ‘n lang tydperk en deur verskillende navorsers gedoen is. Die buigmodel word nogtans as voldoende beskou vir die ontwerpdoeleinde van SHCC.

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ACKNOWLEDGEMENTS

I would like to thank the following people for their assistance:

Prof Gideon P.A.G. van Zijl, my supervisor, for his guidance, support and insight.

The staff of the laboratory and workshop of the Civil Engineering Department, University of Stellenbosch, for their time and effort in assisting with the experimental work.

My friends and family for their love and support during this research period. Thanks Geoff, Johan and Reenen for your words of encouragement. Thank you Dominique, for your loving support, understanding and patience.

The financial assistance of the National Research Foundation (NRF) towards this research is hereby acknowledged. Opinions expressed and conclusions arrived at, are those of the author and are not necessarily to be attributed to the NRF.

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4.3.1 CAST SHCC AND R/SHCC ...36 4.3.2 EXTRUDED SHCC AND R/SHCC ...38 4.3.3 CAST R/C...39 4.4 RESULTS...40 4.4.1 CAST SHCC AND R/SHCC ...42 4.4.2 EXTRUDED SHCC AND R/SHCC ...43 4.4.3 CAST R/C...45 4.5 DISCUSSION...47 4.5.1 GENERAL...47 4.5.2 MECHANICAL RESPONSE...49 4.5.2.1 Unreinforced specimens...50 4.5.2.2 Reinforced specimens ...52

5 MECHANICAL CHARACTERISTICS OF EXTRUDED SHCC ...58

5.1 TENSILE MECHANICAL TESTS...58

5.1.1 TEST SET-UP...58

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5.1.3 RESULTS...62

5.1.4 DISCUSSION...66

5.2 COMPRESSIVE MECHANICAL TESTS...68

5.2.1 TEST SET-UP...69

5.2.2 EXPERIMENTAL TEST PROGRAM...71

5.2.3 RESULTS...72

5.3 ELASTIC MODULUS OF SHCC ...76

5.3.1 TEST PROGRAM AND RESULTS...76

6 BENDING MODEL FOR SHCC ...82

6.1 MODEL DESCRIPTION...82

6.1.1 COMPRESSIVE AND TENSILE STRESS-STRAIN MODEL...83

6.1.2 STEEL STRESS-STRAIN MODEL...86

6.2 BENDING CALCULATIONS AND RESULTS...87

6.3 DISCUSSION...89

7 CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE RESEARCH...92

7.1 CONCLUSIONS...92

7.1.1 COMPARATIVE STUDY...92

7.1.2 MECHANICAL CHARACTERISTICS OF EXTRUDED SHCC ...93

7.1.3 BENDING MODEL...94

7.2 FUTURE RESEARCH...94

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x

LIST OF FIGURES

Figure 2.1: The tensile stress-strain behaviour of cement-based composites...6

Figure 2.2: Steady-state crack analysis presents two crack propagation scenarios: (a) The Griffith crack, where the fibres slip out or rupture in the mid-crack section where δm exceeds δp. (b) The steady-state flat crack, where the fibres remain intact as the crack propagates under a constant σss, with δss < δp [Li 2002]. ...6

Figure 2.3: Crushing failure in a compressed SHCC specimen [Fantilli et al. 2007]...7

Figure 2.4: Grading of F95 sand, a proportioned blend of Philippi (dune) sand and crusher dust...14

Figure 2.5: Schematic representation of piston-driven extrusion. Material is loaded into the loading chamber and pushed forward by a piston through a transition zone and out through the die. ...15

Figure 2.6: Three-point flexural bending in the (a) Longitudinal and the (b) Orthogonal direction [Visser 2005]...17

Figure 2.7: Typical load-deformation, tension-stiffening response of R/SHCC in comparison to R/C and the load-deformation response of bare steel [Fischer and Li 2002]. ...20

Figure 2.8: Schematic of crack formation and internal stresses in R/C and R/SHCC composites [Fischer and Li 2002]...21

Figure 3.1: Piston-driven plate extruder developed by De Koker [2004]...23

Figure 3.2: The different zones within an extruder (a) and the free-body diagram for an infinitesimal sized element of the loading chamber and die (b). ...26

Figure 3.3: (a) A diagram of a single slope transition zone with the height reducing and (b) & (c) free-body diagrams of an infinitesimal sized element of a single slope transition...28

Figure 3.4: An isometric view of the new piston-driven extruder...32

Figure 4.1: Beam moulds for the casting of bending specimens. (a) & (b) Wooden moulds for reinforced specimens and (c) standard steel moulds for unreinforced specimens...34

Figure 4.2: (a) The new piston-driven extruder with Instron actuator and (b) reinforcing steel ready to be used during extrusion. ...35

Figure 4.3: Three-point flexural bending test set-up and bending specimen...36

Figure 4.4: Mixing procedure with time shown for each step of mixing cast SHCC...37

Figure 4.5: Mixing procedure with time shown for each step of mixing extrusion SHCC. ...38

Figure 4.6: (a) Extrusion process in progress and (b) an extruded specimen just after extrusion...39

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Figure 4.8: Three-point flexural bending test results of cast SHCC and R/SHCC...42

Figure 4.9: Three-point flexural bending test results of extruded SHCC and R/SHCC. ...44

Figure 4.10: Three-point flexural bending test results of cast R/C vs. that of cast and extruded SHCC. ...46

Figure 4.11: Three-point flexural bending test results of cast R/C vs. that of cast and extruded R/SHCC. ...46

Figure 4.12: Surface flaws on extruded specimens...48

Figure 4.13: Comparison of (a) ductility, (b) first cracking force and (c) ultimate resistance. ...49

Figure 4.14: (a) Bending moment diagram and (b) shear force diagram for three-point bending tests. ...49

Figure 4.15: Crack formation and cracking pattern of a typical cast SHCC beam...50

Figure 4.16: Graphical representation of the area of multiple cracking in a flexural test [Boshoff 2006]. ...51

Figure 4.17: Crack formation and cracking pattern of typical extruded SHCC beam...51

Figure 4.18: Illustration of principal stress rotation and crack alignment using a Mohr circle. ...53

Figure 4.19: Crack formation and cracking pattern of a cast R/SHCC beam...53

Figure 4.20: Crack formation and cracking pattern of a typical extruded R/SHCC beam. ...54

Figure 4.21: Diagram of force equilibrium of a reinforced concrete beam at the ultimate limit state [SABS 0100 1994]...56

Figure 4.22: Crack formation and cracking pattern of a typical cast R/C beam...57

Figure 5.1: The dimensions of the flat dog bone specimen. ...59

Figure 5.2: The steel mould used for casting of flat dog bone specimens with the two removable studs...59

Figure 5.3: The tensile test set-up in the Zwick Z250 and the aluminium frame with the two LVDT’s that were used to measure the deformation over the gauge length...60

Figure 5.4: Direct tensile response of SHCC specimens cut from extruded plate specimens. ...62

Figure 5.5: Direct tensile response of SHCC specimens cut from the bottom of extruded beam specimens...63

Figure 5.6: Direct tensile response of SHCC specimens cut from the middle of extruded beam specimens...63

Figure 5.7: Direct tensile response of cast SHCC specimens in comparison with the direct tensile responses of the various extruded SHCC specimens...64

Figure 5.8: Comparison of (a) first cracking strength, (b) ultimate strength and (c) ductility. ...66

Figure 5.9: Cast SHCC tensile response versus A/B ratio [van Zijl 2005]...67

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Figure 5.11: Dimensions of the core specimens. ...69

Figure 5.12: Copper mould used for casting SHCC cores...69

Figure 5.13: The compressive test set-up in the Zwick Z250 and the aluminium frame with the two LVDT’s that were used to measure the strain over the gauge length. ...70

Figure 5.14: Direct compressive response of cast SHCC cores (height: 100 mm; diameter: 50 mm). ...72

Figure 5.15: Direct compressive response of extruded SHCC cores (height: 100 mm; diameter: 50 mm). ...73

Figure 5.16: E-modulus of cast and extruded SHCC computed from tensile and compressive responses. ...78

Figure 5.17: Influence of aggregate content on the E-modulus of cast SHCC [van Zijl 2005]...79

Figure 5.18: Influence of aging (A/B = 0.5) on the E-modulus of cast SHCC [van Zijl 2005]. ...80

Figure 6.1: Diagram of the strain and stress distribution of an SHCC cross section...83

Figure 6.2: Tensile stress-strain model for SHCC material...84

Figure 6.3: Tensile stress-strain curve fitting for (a) cast SHCC and (b) extruded SHCC...85

Figure 6.4: Compressive stress-strain model for SHCC material...85

Figure 6.5: Compressive stress-strain curve fitting for (a) cast SHCC and (b) extruded SHCC...86

Figure 6.6: Tensile stress-strain model for reinforcing steel. ...87

Figure 6.7: Computed resistance moment of thin cast and extruded SHCC plates (15 mm X 70 mm). ...88

Figure 6.8: Computed resistance moment of cast and extruded SHCC and R/SHCC beams (100 mm X 100 mm)...88

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LIST OF TABLES

Table 2.1: Properties of PVA-fibre [Horikoshi et al. 2006]...10

Table 4.1: Cast SHCC mix design [Boshoff 2006]...36

Table 4.2: Extrusion SHCC mix design [Visser 2005]...38

Table 4.3: R/C mix design ...39

Table 4.4: Ultimate compressive strengths of cast concrete and SHCC...41

Table 4.5: Flexural characteristics of cast SHCC specimens...43

Table 4.6: Flexural characteristics of cast R/SHCC specimens...43

Table 4.7: Form retention measurements for the extruded SHCC specimens ...44

Table 4.8: Form retention and steel cover measurements for the extruded R/SHCC specimens ...44

Table 4.9: Flexural characteristics of extruded SHCC specimens...45

Table 4.10: Flexural characteristics of extruded R/SHCC specimens ...45

Table 4.11: Flexural characteristics of cast R/C ...47

Table 5.1: Extrusion SHCC mix design [Visser 2005]...61

Table 5.2: Test program for the uni-axial tensile tests...62

Table 5.3: Tensile characteristics of SHCC specimens cut from extruded plate specimens ...65

Table 5.4: Tensile characteristics of SHCC specimens cut from the bottom of extruded beam specimens ...65

Table 5.5: Tensile characteristics of cast SHCC specimens ...65

Table 5.6: Ultimate 14day compressive strengths of cast and extruded SHCC ...74

Table 5.7: Ultimate 14day compressive strengths of cast and extruded SHCC with various aspect ratios...75

Table 5.8: E-modulus values for cast and extruded SHCC computed from uni-axial tensile tests. ...77

Table 5.9: E-modulus values for cast and extruded SHCC computed from uni-axial compressive tests. ...77

Table 6.1: Parameter values for the tensile stress-strain model for cast and extruded SHCC...84

Table 6.2: Parameter values for the compressive stress-strain model for cast and extruded SHCC ...86

Table 6.3: Parameter values for the tensile stress-strain model for the reinforcing steel. ...87

Table 6.4: Ultimate moment resistance and ultimate applied forces for thin SHCC plates ...89

Table 6.5: Ultimate moment resistance and ultimate applied forces for SHCC and R/SHCC beams ...89

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NOMENCLATURE

A/B Aggregate-Binder ratio Ac Cross sectional area of concrete

As Cross sectional area of steel reinforcing

c Crack spacing

C.O.V. Coefficient of Variation

E Elastic modulus

Ea Aggregate elastic modulus

Ec Elastic modulus of the composite

Em Matrix elastic modulus

FA Fly Ash

FA/B Fly Ash-Binder ratio

fcu Ultimate compressive strength

Ffc First cracking force

Fmax Maximum force

FRC Fibre Reinforced Concrete ƒt Tensile strength

ƒt,ƒ Tensile strength of fibres

ƒtu Ultimate tensile strength

fy Yield strength

Gƒ Fracture energy

GGCS/B Ground Granulated Corex Slag-Binder ratio GGCS Ground Granulated Corex Slag

HCC Hybrid Concrete Construction

HPFRCC High Performance Fibre Reinforced Cement Composite Lm Length of multiple cracking zone

Mmax Maximum moment

MOR Modulus of Rupture Ø Diameter

OPC Ordinary Portland Cement

p Porosity

P(x) Horizontal pressure at point x PVA Poly Vinyl Alcohol

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R/SHCC Reinforced Strain Hardening Cement-based Composites RD Relative Density

SHCC Strain Hardening Cement-based Composites

SP Superplasticiser

StdDev Standard Deviation

V Shear force

Va Aggregate volume

VA Viscous agent

vc Shear stress capacity

Vf Fibre volume

Vm Matrix volume

Vp Paste volume

W/B Water-Binder ratio γm Material factor

δm Crack mouth width

δp Ultimate fibre deformation

δss Steady-state crack width

ε0 Strain at pre-stress σ0

εc Compressive strain

εcu Ultimate compressive strain

εt Tensile strain

εtmax Maximum tensile strain

εtu Ultimate tensile strain

εu,ƒ Ultimate fibre elongation

η Efficiency coefficient μ Friction coefficient

σ0 Pre-stress

σc Compressive stress

σca Standardized compressive strength

σcu Ultimate compressive stress

σƒc First cracking strength

σss Steady-state stress

σt Tensile stress

σtf Tensile first cracking stress

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xvi τ(x) Friction surface / shear stress at point x τmax Maximum shear stress

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1 INTRODUCTION

Over the centuries, concrete has been used as a reliable, fairly durable building material. Two of the main advantages of concrete are that it has a high compressive strength and that it can be cast on the construction site into virtually any size and shape. The shortcomings of concrete are becoming more and more prominent in this day and age with the emphasis of design moving towards optimised economical designs. The most prominent disadvantages of concrete are the brittleness during failure and the low tensile strength, which is about one tenth of the compressive strength. The low tensile strength is compensated for with the use of reinforcing steel, but cracking still occurs during the normal use of reinforced concrete. These cracks lead to durability problems as water penetrates the concrete through the cracks and causes the corrosion of the reinforcing steel which in turn leads to structural degradation.

SHCC (Strain-Hardening Cement-based Composite) is a type of HPFRCC (High Performance Fibre Reinforced Cement Composite) that has been engineered to overcome the weaknesses of ordinary concrete as mentioned above. It shows high ductility as it can resist the full tensile load at a strain of more than 3% compared to concrete which fails on average at a strain of 0.01% in tension. When using SHCC instead of ordinary concrete the strain capacity increases more than 300 times. This leads to a high energy absorbing capacity of SHCC.

During the increase of strain under tensile loading of SHCC, a constant strain-hardening effect is found and many, closely spaced, micro-cracks are formed in the material. These fine, multiple cracks reduce the problem of water penetration and thus improve the durability of the structures. SHCC is more expensive than ordinary concrete, but if the superior material characteristics are exploited, it becomes a competitive material in the building industry. The advantages of SHCC may be exploited in several applications:

Seismic loading application: Due to the high ductility and flexibility of SHCC it can be used in designated areas in a multi-storey structure to withstand seismic loadings.

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Thin membrane and shell members: Due to the high ductility, thin membrane members, i.e. less than 15 mm thick, can be made e.g. pipes, tiles, etc.

Repair material: Due to the high strain capacity and energy absorption, SHCC is ideal to be used as repair material, especially for bonded overlays.

Durable structural application: Due to the fine, multiple cracking phenomenon of SHCC it is an ideal cover material for reinforcing steel in R/C elements.

Composite application: Applied in combination with R/C, for instance as a permanent formwork shell for (reinforced) concrete members, SHCC adds the benefits of construction time saving and shear steel reduction, as SHCC has a higher shear resistance than ordinary concrete.

South Africa is currently facing a shortage of engineers, technicians and other skilled workers in the construction industry. The shortage of skills clearly places high demands on designers and contractors to provide services and to realise projects in ever reducing time periods and at lower cost. These conditions, in a growing economy, augmented by the shortage of manpower, make it increasingly difficult to maintain quality of construction in an industry where mistakes can lead to disastrous consequences. Hybrid Concrete Construction (HCC) is a type of construction where a combination of prefabricated concrete elements and cast in-situ concrete is used and is seen as a possible solution to these problems that the South African construction industry is facing. This is done by using the optimal combination of both types of concrete construction by considering time and cost reduction, but with increased quality. SHCC is a construction material suitable to this type of construction and can be used for various applications, as mentioned above.

Extrusion is a special type of manufacturing method by which structural elements with complex geometrical cross section can be made to virtually any length, permitting for space restrictions. This type of manufacturing method of SHCC fits in well with the idea of HCC. Not much research has been done on the extrusion of SHCC, especially on piston-driven extrusion. This thesis document reports on research that was done to determine the mechanical properties of extruded SHCC with the structural use thereof in mind. It also contains a description of the new piston-driven extruder that was designed to manufacture extruded SHCC and R/SHCC (steel reinforced SHCC) as well as mechanical tests that were conducted on both types of composites. In addition, a simple bending model is presented for the design/analysis of (R/)SHCC, which uses these characterised properties of SHCC.

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In Chapter 2 a theoretical background is given on SHCC, its typical mechanical properties and the properties of its constituent material. The extrusion of SHCC is also covered.

In Chapter 3 a description of the design of the new piston-driven extruder facility is given along with a description of the extruder itself.

In Chapter 4 a comparative study of cast and extruded SHCC and R/SHCC as well as Reinforced Concrete (R/C) is undertaken. This is done by comparing the mechanical properties of the composites in flexure.

In Chapter 5 the tensile and compressive mechanical properties of extruded SHCC are investigated and compared to that of cast SHCC. This is done by comparing responses to uni-axial tests on extruded SHCC.

A simple bending model for SHCC, based on its uni-axial properties as determined in the previous chapter, is introduced in Chapter 6. The results obtained with this model are also compared with experimentally determined results.

Finally, in Chapter 7 conclusions, based on the research study reported in this thesis document, are drawn and recommendations are made for possible future research studies.

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2 BACKGROUND OF SHCC AND

EXTRUSION

2.1 INTRODUCTION

Concepts about the development of SHCC as a durable building material will be presented in this section. Attention is given to the unique material properties and constituents used to achieve the material behaviour. The concept of the extrusion of SHCC as well as the development thereof thus far is also introduced.

The focus of this literature study will be directed towards characterising SHCC, its constituents and properties. Secondly it will focus on the extrusion of SHCC, the requirements for extrusion and the characteristics of extruded SHCC. The third section contains a discussion of the benefits and applications for reinforced SHCC (R/SHCC).

2.2 PROPERTIES OF SHCC

2.2.1 MATERIAL PROPERTIES

Most of the properties that are discussed in this section are based on work that has been done with SHCC prepared by casting in moulds, since casting has been the preferred way of manufacturing for SHCC thus far. The basic concepts do however apply to SHCC in general. The extrusion of SHCC specifically, will be dealt with in another section of this chapter.

2.2.1.1 Tensile properties

Most of the research that has been done on SHCC has been focussed on the tensile behaviour and tensile characteristics of SHCC. The most fundamental mechanical property of SHCC is the ductile tensile behaviour after the occurrence of the first crack. The fundamental difference between SHCC and ordinary fibre reinforced concrete (FRC) is the strain-hardening response that accompanies the

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ductile behaviour of SHCC. FRC displays a decline in strength with an increase in strain beyond the occurrence of the first crack, which is referred to as strain-softening. Strain-hardening behaviour on the other hand is a behaviour that displays an increase in post cracking strength, accompanied by the formation of multiple cracks, not always visible to the naked eye. Multiple cracks are the only means by which a rigid cement-based material can manifest strains larger than elastic strain, with an increase in strength. Note that the notion of strain is used in a macroscopic sense. A non-uniform deformation does occur, with higher local deformation in the vicinity of cracks due to lower stiffness in these regions of bridged cracking. Nevertheless, this non-uniformity is much less pronounced in the case of multiple fine cracks, than in R/C or FRC with localised wide cracks. This multiple cracking is accomplished by means of the capacity of fibres to bridge and transfer the applied stresses over the matrix cracks. As for any material under tensile load, the matrix cracks at the weakest point. If no fibres are present in the matrix, the material would fail completely. In the case of FRC, the crack bridging capacity is less than the cracking strength of the concrete, thus the crack widens and the load is reduced if the deformation is increased, i.e. strain-softening occurs. In the case of SHCC, however, the fibres bridging the crack in the matrix have the capacity to sustain the load. In the case of SHCC however the fibres have the capacity to sustain the load. If the deformation is then increased, the load will increase and the matrix will crack at the next weakest point which is stronger than the first cracking point. Thus an increase of the stress is found, i.e. strain-hardening occurs. The fibres implemented for this research are poly vinyl alcohol (PVA) fibres. Only when strain-hardening reaches its peak at about three to six percent strain does SHCC start to display a strain-softening trend, also exhibited by FRC. At this point a localising crack forms at one of the multiple cracks and strain-softening starts. The cumulative interfacial bond strength between PVA fibres and the matrix over a section controls the ultimate tensile strength of SHCC (ƒtu). A stress-strain curve similar to that of a ductile metal is achieved. This is illustrated in

Figure 2.1 along with a comparison between FRC and ordinary concrete.

Crack widths in the matrix are controlled by the elastic strain and slip of fibres bridging the matrix cracks. The ability of SHCC to control crack width, occurs in the envelope starting from first cracking strength (σƒc) and ending at the ultimate strength (ƒtu).SHCC, like concrete is a dynamic

material in which the hydration process continues for very long periods until most of the free water is incorporated in hydration products. When cracks occur, as they do in all cement-based products, surfaces are exposed to water particles. The ability of SHCC to limit crack widths induces the phenomenon of autogenous healing (formation of CaO-SiO2-H2O over fine cracks) [Illston and

Domone 2001]. This self-healing ability of SHCC, which is currently studied intensely [Yang et al. 2005, Li and Yang 2007], is an added benefit and can be exploited for durability purposes.

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Strain

Stress

Concrete

SHCC

FRC

post-crack region _{strain-hardening}

strain-softening

σtf

σtu

Figure 2.1: The tensile stress-strain behaviour of cement-based composites.

Another important consideration when developing a ductile SHCC, is the fracture energy (Gƒ) of

the cementitious matrix. Gƒ is an attribute influenced by the micro-structure [Li 2003]. It is

measured as the amount of energy dissipated per unit crack surface. High Gƒ values for the material

matrix adversely affect the strain-hardening property. It may lead to non-uniform loading of fibres in a crack, which may cause consecutive, progressive fibre breakage or fibre pull-out, instead of the crack extending in its length. Figure 2.2 illustrates two types of cracks that might occur in SHCC, namely the Griffith crack and the steady-state crack [Li 2003].

δss < δp δm > δp σss σss σss σss broken fibres (a) (b)

Figure 2.2: Steady-state crack analysis presents two crack propagation scenarios: (a) The Griffith

crack, where the fibres slip out or rupture in the mid-crack section where δm exceeds δp. (b) The

steady-state flat crack, where the fibres remain intact as the crack propagates under a constant σss,

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Griffith crack the fibres slip out tion, where the crack mouth opening

es

e behaviour of SHCC has been limited to the determination of the

ecentl

In a or rupture in the midsec

(δm) exceeds the fibre deformation at its ultimate resistance (δp). The fibres for the steady-state

crack remain intact as the crack propagates under a constant steady-state stress (σss), with the

steady-state crack opening (δss) less than δp. One way of manipulating Gƒ in cement-based

composites is to vary the aggregate content. Higher aggregate content leads to more complex crack paths and associated high Gƒ.

2.2.1.2 Compressive properti

Research based on the compressiv

ultimate compressive strength thereof and to some extent the determination of the compressive stress-strain response of SHCC. For the most part the compressive response of SHCC has been modelled using a phenomenological approach [Kabele 2000 & 2007, Kanda et al. 2005], where a curve is fitted to the observed response. The compressive response of SHCC up to the ultimate compressive stress can be characterized as linear elastic until the ultimate compressive stress is reached with the slope of the curve determined by the E-modulus of the composite, although the stress-strain curve tends to deviate from its linearity as it reaches the ultimate stress. SHCC displays a strain-softening post-peak response which can be utilized in the post-peak response of structural members.

Figure 2.3: Crushing failure in a compressed SHCC specimen [Fantilli et al. 2007]. H

w

σc = σcu +Δσc

R y, efforts have been made to characterize the strain softening part of the compressive

strain curve after the peak or ultimate stress is reached [Fantilli et al. 2007], in other words after localized damage has occurred. At this stage, the progressive sliding of two blocks of cement-based material is evident [Figure 2.3]. According to Fantilli et al. this part of the stress strain curve can be represented by the following equation:

H w E w F cu cu c + − ⋅ − =ε σ [1 ( )] ε for ε_c >ε_cu (2.1)

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where εcu is the strain at the ultimate compressive stress σcu; w is the inelastic displacement of the

pecimen as defined in Figure 2.3; H is the height of the specimen; E is the elastic modulus of the

ty of Stellenbosch, up to now, has een limited to the determination of the ultimate compressive strength of SHCC [Stander 2007]. It

ear properties

The main focus of research based on the shear behaviour of SHCC has been on heavily steel t replacing concrete in shear-critical structural elements [Shimizu et al.

f the research as to develop a shear test for SHCC and to characterize the shear behaviour of this SHCC. The s

composite; and F(w) is a non-dimensional function which connects w and the relative stress σc / σcu

during softening. The function F(w) is considered to be a material property and should be determined for the specific SHCC material under consideration.

Research on the compressive behaviour of SHCC at the Universi b

was found, like in other studies, that the tensile strength is approximately 1/10 of the compressive strength.

2.2.1.3 Sh

reinforced SHCC aimed a

2006, Kabele 2005, Suwada and Fukuyama 2006]. Efforts have however been made to characterize the shear behaviour of SHCC itself [Li et al. 1994, Shang 2006]. The research by Li et al. was done on SHCC beams containing 2% by volume polyethylene fibres where this material was subjected to an Ohno-type shear beam test, which creates a pure shear plane in the centre of the specimens. While these tests demonstrated the contribution of shear resistance by SHCC to the composite beams, they could however not be used for the objective determination of the shear characteristics of SHCC, because of the use of longitudinal reinforcement to prevent flexural failure.

The research done by Shang was done on SHCC containing PVA-fibres and the aim o w

shear test that was developed is based on the Iosipescu shear test [Iosipescu 1967] that is used for shear testing of metals, fibre reinforced plastics and wood. The SHCC that was tested displayed a ratio between the ultimate shear strength and the first cracking strength of 2.25 and the ratio between the ultimate shear strength and the ultimate uni-axial tensile strength of 1.5. The SHCC forms multiple micro cracks in shear, whereby the true nature of SHCC, as also exhibited in uni-axial tension, is mobilised. The cracks develop at an angle greater than 45° (60°-65°), dominated by the principal stress direction, which in turn depends on the compressive/tensile strength ratio. This can only be realised if sufficient ductility in tension allows the tensile resistance to be maintained at deformations beyond the first cracking deformation, to enable the compressive resistance to be

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mobilised. The final failure of the SHCC in shear is when these diagonal micro-cracks are connected with vertical cracks.

2.2.1.4 Elastic Modulus

One hindrance for the structural use of SHCC is its low Elastic Modulus (E-modulus). In various research studies done at the University of Stellenbosch on SHCC [Shang 2006 and Boshoff 2006] the E-modulus of SHCC has been determined to be between 7.5 and 10 GPa. These values were determined by applying a secant method to compute the E-modulus from direct tensile responses at stress levels one third of the first cracking stress and a low pre-stress (σ0 = 0.1 N/mm2) and the

corresponding strains as follows:

0 ) 3 1 ( 0 3 1 ε ε σ σ σ − − = tf tf c E (2.2)

The E-moduli of between 7.5 and 10 GPa were computed for cast SHCC specimens that were cured for 14 days and that were subsequently tested at the age of 14 days. Similar tests done on a SHCC at 28 days showed that the E-modulus increases [van Zijl 2005] and may roughly be double at this higher age [Li et al. 1995]. This would suggest that E-modulus of SHCC at 28 days would be roughly between 13 and 20 GPa. This is still significantly less than the 28GPa that is suggested for ordinary concrete with equivalent compressive strength (σcu = 30 MPa) by the SABS 0100-1 code

[2000]. It has however been shown that by increasing the aggregate content of a SHCC mix, the E-modulus is also significantly increased [van Zijl 2005].

2.2.2 MATRIX CONSTITUENT PROPERTIES

2.2.2.1 General

SHCC utilises the same material ingredients as ordinary FRC’s, such as water, cement, sand, fibres and other common chemical additives, but it is rather the optimal combination of these ingredients, based on micromechanical considerations, that affords SHCC its unique ductile, tensile strain-hardening behaviour. Course aggregates are not used as they tend to increase the fracture toughness which negatively affects the unique ductile behaviour of the composite. Larger aggregate particles also prevent effective fibre dispersion and crack bridging. Unlike some high performance FRC’s, SHCC does not utilise large amounts of fibres. In general 2% or less by volume of discontinuous fibres is adequate. This relatively small amount of fibres with its short fibre length contributes to the fact that the mixing procedure is similar to that of ordinary concrete.

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Hardened SHCC consists of three main components, namely fibres, cement-based matrix and the fibre-matrix interface. Proportioning of each component with the correct mechanical and geometric properties is necessary to attain the unique ductile behaviour. Material design of SHCC is guided by micromechanical principles. Admixtures are incorporated into the mix design, because they enhance the properties of the SHCC in the fresh state, which in turn result in a beneficial hardened state. The beneficial condition is defined by the uniform spread of constituents, causing a resemblance to isotropic material properties. Uniformity of constituent rheology is imperative for the functioning of micromechanical properties.

2.2.2.2 Fibres

Several different types of fibres exist which can be used in SHCC. As stated previously, PVA-fibres were used in this research. These fibres were chosen for their relative high tensile strength (ƒt) and

E-modulus (E). Thereby the fibre breakage at the crack regions, which will lead to premature brittle behaviour of the composite, can be avoided or restricted. A fibre length of 12mm was chosen to ensure better fibre dispersion and a more workable material in its fresh state. Longer fibres tend to coagulate and form fibre balls. The ƒt, E-modulus and other properties of these PVA-fibres are

shown in Table 2.1.

Table 2.1: Properties of PVA-fibre [Horikoshi et al. 2006]

Type Diameter [mm] Length [mm] ƒt,ƒ[GPa] E [GPa] εu,ƒ [%]

PVA-REC15 0.04 12 1.6 37 6

2.2.2.3 Admixtures

Admixtures are chemicals that are added to the concrete immediately before or during mixing and significantly alter its fresh, early age or hardened state to ensure economic or physical advantages. Normally only small quantities are required, typically 0.1 to two percent by weight of the binder.

Superplasticiser (SP)

Known as workability aids, it increases the fluidity or workability of a cement paste or concrete. They are also referred to as high-range water reducers. It has a high molecular weight and is manufactured to high standards of purity and can therefore achieve substantially greater primary effects without significant side-effects.

The addition of fibres and methyl cellulose lead to an increase in viscosity, thus needing an additive (SP) to improve the workability. The SP also serves to ensure a workable mix in the case of

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extrusion where a highly viscous, dough-like mix is required, which is obtained with a low water-binder ratio.

The mode of action induced by SP is purely physical and a combination of mutual repulsion and steric hindrance between cement particles, creating less friction when the particles move. The behaviour of any particular combination of SP and cement will depend on several factors other than the admixture type, including the cement composition, the cement fineness and the water-binder (W/B) ratio.

Substantially increased performance can be obtained if the SP is added a short time (1-2 minutes) after the initial contact between the mix water and the binder [Illston and Domone 2001]. When SP and mix water are added simultaneously, a significant amount of SP is incorporated into the rapid (CaO)3·Al2O3 / gypsum reaction, hence reducing the amount of available to attain workability. The

SP action only occurs for a limited time period, usually half an hour to a few hours.

The SP that was used in this research is a product of Chryso, namely Optima 100, with a relative density (RD) of 1.2.

Methyl Cellulose (Viscous Agent)

Chryso Aquabeton ZA is a powder additive necessary to prevent segregation and wash out of the fresh concrete. Also known as viscous agent (VA), it causes an increase of intermolecular shear force of the fresh concrete. Thus it acts as a dispersion agent and assists in the uniform dispersion of the fibres in the mix. VA is also an essential component of an extrusion mix, as it prevents segregation in the form of water being squeezed out under the high extrusion pressure. The addition of the Chryso Aquabeton ZA will tend to reduce the workability of a concrete or cement-based mix. This has to be taken into account when designing a SHCC mix. Workability is compensated for by the use of the correct water-binder ratio and an optimal amount of SP.

2.2.2.4 Binder

The binders considered in this research are Ordinary Portland Cement (OPC – CEM I 42.5N), Fly Ash (FA) and Ground Granulated Corex Slag (GGCS). For the extrusion mixes the binder consisted of a 50:50 combination of FA and OPC, while the composition of the binder for the cast mixes were 50% FA, 45% OPC and 5% GGCS. The cast mix is a standard mix design that was used by Boshoff [2006] in his research at the University of Stellenbosch of which some results were used in the current research for comparison purposes. The slight difference between the binder content of the

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two mix designs were deemed minor for the purposes of this research, but will be adequately discussed in the evaluation of results in this thesis.

Ordinary Portland Cement (CEM I 42.5N)

CEM I 42.5N is a hydraulic cement consisting essentially of hydraulic calcium silicates. Since the cement is composed of a heterogeneous mixture of several compounds, the hydration process consists of reactions of the anhydrous compounds with water, occurring simultaneously. As the hydration reaction of cement compounds is exothermic, the compounds of cement are none-equilibrium products of high temperature reactions and are therefore in a high-energy state. The cultivated heat of hydration could lead to cracks in some applications and affect the structural strength and durability. The 42.5 in the name corresponds to the strength in MPa achieved at 28 days after curing with a W/B ratio of 0.5. The letter N for normal depends on the strength after 2 days, in this case equal to or more than 10MPa.

Fly Ash (FA)

FA is collected, by electrostatic precipitator, form the flues of power stations that burn finely ground coal. Being a by-product of an industrial process, it has cost advantages and is readily available in the inland regions (mostly Gauteng) of South Africa, but not at the coastal regions. FA is a pozzolanic material which, when mixed with Portland cement and water, reacts with calcium hydroxide (product of cement hydration) to produce CaO-SiO2-H2O gel. Other reasons for

the use of this product are that it gives a variety of useful enhancements to the concrete product. The spherical shape of the particles causes an increase in mix workability. The relative density is also less than that of cement, and therefore the substitution on a weight-by-weight basis will lead to an increased volume. It increases the durability of cement-based products in the sense that it increases the density of the material and improves the mechanical behaviour, in particular in extruded products of SHCC. The addition of FA enhances the ductile behaviour of extrusion composites with long fibres [Shah and Peled 2003]. The FA changes the failure mode form fibre breakage to fibre pull-out and thus enhances the ductile behaviour. The extrusion process provides a strong fibre-matrix bond due to the decreased porosity of the extrudate and the addition of FA reduces the bond strength and changes the mode of failure.

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FA also limits the heat exerted during hydration because it prolongs the hydration process. The type of FA implemented in the research was Dura-Pozz, a graded class-F FA product provided by Ash Resources, South Africa.

Ground Granulated Corex Slag (GGCS)

The Corex slag considered in this research is a by-product of the steel manufacturing process at Saldanha steel plant in the Western Cape region of South Africa. Slag is a latent hydraulic binder in that it hardens very slowly in water, but becomes much more reactive when activated by alkalinity of calcium hydroxide, a product of cement hydration. The introduction of Corex slag as a cement extender does not only improve the durability and workability of cement-based materials, but also reduces the cost of the material.

The RD is also less than that of cement and therefore, the substitution on a weight-for-weight basis will lead to a volume increase. The particles are finer than cement particles, which results in better workability, lower bleed rates and shorter setting times than conventional slag [Mackechnie et al. 2003]. Increased durability is achieved via an increased density of the composite. Slag also limits the heat of hydration.

2.2.2.5 Fine Aggregate

An important aspect of a cement-based mix is to ensure the uniform spread of different aggregate grain sizes. This spread of the grain sizes is referred to as the grading of the aggregate. It is important to ensure that the particle sizes are not all the same and that they yield a dense packing of the aggregate, which in turn cultivates workability and durability.

A F95 grading is a fine grading and is used to ensure a good spreading of the fine sand particles, ensuring that the role of the aggregate is optimal. The fine grading ensures a lower matrix fracture toughness, which conforms to micromechanical models of strain-hardening. This suggests that a matrix with lower toughness (in comparison to concrete) should require a smaller amount of fibres to make the transition from brittle to pseudo-strain-hardening mode of failure. The process of grading the sand is expensive, but it is used to ensure the occurrence of the multiple cracking phenomena which yields high ductility and durability. The RD of the sand used in this research is 2.7. It is blended sand prepared by sieving and proportioning local Philippi (dune) sand. Since this sand lacks the required fine particles, crusher dust was used to complete the required F95 grading [Figure 2.4].

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0 10 20 30 40 50 60 70 80 90 100 1 10 100 1000

Particle size [um]

% t

h

rough sieve

Crusher dust Philippi sand

Figure 2.4: Grading of F95 sand, a proportioned blend of Philippi (dune) sand and crusher dust.

2.3 EXTRUSION OF SHCC

2.3.1 INTRODUCTION

Extrusion can be described as a plastic moulding process whereby structural elements are formed under high shear and high compressive forces. An extruder is a machine that forces material through a die by applying pressure. It produces products with a constant cross section and preferably a high symmetry. Extrusion has been used successfully by the ceramic, plastics and even metal and aluminium industries for many years already to produce products such as pipes, facades, balustrades and structural steel members with complex sections. More recently, extrusion is also being used by the prefabrication concrete industry to produce hollow-core slabs and lintels. In this case the concrete is extruded on a flat bed with the extruder moving along the bed. The extrusion of fibre reinforced concretes is still a relatively new concept with various potential applications.

There are two basic types of extrusion that can be used for the extrusion of SHCC, namely wet-mix extrusion like that used by Stang [1999] or dry-mix extrusion. In the wet-mix method extra water is added to the mix to create a flowable, workable mix in the fresh state. The excess water is then forced out during extrusion in a consolidation phase. Dry-mix extrusion can be further subdivided into two categories, namely auger-driven extrusion or piston driven-extrusion. Auger-driven

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extrusion uses a screw to convey material toward the extruder die which has a smaller cross section than the screw barrel. Pressure is built up due to this change in cross section, which results in the compaction of the material and the material then being forced through the die. The screw allows material to be constantly fed into the extruder.

Piston-driven extrusion, which is the type of extrusion that is used in this research, uses a piston to force the material forward. Figure 2.5 illustrates a basic piston-driven extrusion set-up.

Die Transition zone

Loading Chamber Piston

Figure 2.5: Schematic representation of piston-driven extrusion. Material is loaded into the loading

chamber and pushed forward by a piston through a transition zone and out through the die. The fresh material is loaded into the loading chamber and it is then forced forward by means of a piston. The transition zone allows for the change from a large cross section to the smaller, desired cross section. It is in the transition zone that pressure builds up due to the difference in cross section and where the compaction of the material takes place. The material then travels through the die which has the same cross sectional shape as the element that is being produced. The material then exits the die, with the required cross sectional shape afforded to it by the die, and can then be cut into the desired lengths and taken to a curing room to harden.

2.3.2 RHEOLOGICAL REQUIREMENTS FOR PISTON-DRIVEN EXTRUSION

In order to be able to extrude SHCC by means of piston-driven extrusion, the SHCC mix needs to be tailored to ensure extrudability. Aspects such as segregation and form retention need to be considered when tailoring SHCC for extrusion and this was done in designing an SHCC mix for extrusion in previous research [Visser 2005].

Unlike casting, where you need a flowable mix, extrusion requires a stiff, dough-like composite in its fresh state to ensure form retention after extrusion. The most efficient way to ensure a mix with

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such a rheology is by reducing the W/B ratio. This is however not sufficient, because such a mix lacks cohesion, for which reason both SP and VA are added. Without fluidity and cohesion in an extrusion mix the material will block in the transition zone and pressure will continue to build up, leading to segregation in the form of water being forced from the mix. The SP helps to ensure that the mix is workable, whereas the VA ensures a stiff cohesive mix and prevents segregation.

2.3.3 CHARACTERISTICS OF PISTON-DRIVEN EXTRUDED SHCC

Through the process of extrusion the material and mechanical properties of SHCC are significantly altered. This is firstly due to the tailoring of the SHCC mix for extrusion and secondly due to the influence of the extrusion mechanism itself. These changes need to be considered when deciding on an application for products manufactured by extrusion and thus they will be discussed in the following section.

2.3.3.1 Influence of tailoring

In order to ensure extrudability of SHCC, in other words to create a stiff, dough-like mix, the W/B ratio of extrusion SHCC is significantly reduced in comparison to the W/B ratio of cast SHCC. This results in a higher aggregate content for the extrusion SHCC. This increase in aggregate content can result in an increased E-modulus and an increased tensile as well as compressive matrix strength for the extrusion SHCC in comparison to that of the cast SHCC [van Zijl 2005].

2.3.3.2 Orientation of fibres

Structural applications for SHCC may be optimized by improving the performance of the composite in the direction of the critical failure mode. This implies that to make full use of the benefits of SHCC, the fibres need to be orientated in the principle direction of rupture, the reason being that the fibres are aligned optimally for resistance of action in that direction. Fibres need to be orientated in the longitudinal direction for uni-axial structural elements such as bars, beams and one-way spanning plates, but there should also be orthogonally or diagonally orientated fibres for bi-axially loaded elements such as pressure pipes or two-way spanning plates.

By optimizing the fibre orientation a larger resistance for the same fibre volume can be realized, or reduced fibre content can be used to achieve the required resistance. Furthermore, the ductility of the composite can be improved by enhancing the multiple cracking phenomena, which produces strain-hardening behaviour.

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It has been proven that the fibre orientation of extruded SHCC containing steel fibres is governed by the extrusion mechanism [De Koker 2004]. This implies that fibres are orientated diagonally (in a helix pattern) for auger-driven extrusion and longitudinally, in the direction of the extrusion force, for piston-driven extrusion. This has also been shown by the author to be true for the extrusion of SHCC containing PVA fibres by means of piston-driven extrusion [Visser 2005]. Three-point bending tests were conducted on specimens cut from the longitudinal as well as the orthogonal direction of extruded plates. This SHCC also contained 12 mm PVA fibres and the specimens had dimensions of 15 mm X 30 mm X 70mm and were tested over a span of 60 mm. A comparison was made between the mechanical responses for both directions, where a good ductile, strain-hardening response indicates good fibre orientation in that direction. Although this is an indirect approach and not just a reflection of the fibre orientation alone, it gives a good indication of fibre orientation. The results indicated that the fibres are predominantly orientated in the longitudinal direction of extrusion and that the fibres are poorly orientated in the orthogonal direction. This was reflected by the ductile, strain hardening response of the longitudinal specimens which formed multiple cracks in comparison to the orthogonal specimens which had a brittle response and formed one localized crack. The results of these tests are given in Figure 2.6 below.

0 200 400 600 800 1000 1200 1400 0 0.5 1 1.5 2 2.5 3 Deflection [mm] F o rc e [N ] 0 200 400 600 800 1000 1200 1400 0 0.5 1 1.5 2 2.5 3 Deflection [mm] F o rc e [N ] (a) _(b)

Figure 2.6: Three-point flexural bending in the (a) Longitudinal and the (b) Orthogonal direction

[Visser 2005].

It is the author’s view that the orientation of the fibres takes place in the extruder transition zone and to an extent in the die. When fibres are dispersed in an infinitely large volume of concrete, they are expected to be randomly orientated, with equal opportunity to be orientated in different directions in space. In the presence of parallel boundaries where the distance between sides are relatively small in comparison to the fibre length, the fibres tend to orientate more two-dimensionally near the boundaries. The boundary effect of the sides of the extruder along with the

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high extrusion forces in the direction of material flow in the transition zone and die of the extruder orientates the fibres in the longitudinal direction.

2.3.3.3 Densification and interfacial bond between fibres and the matrix

The performance of SHCC to a large extent depends on the properties of the fibres and the matrix, and those of the fibre-matrix interface. The role of fibres and that of the individual matrix constituents has already been discussed. Here the role of the fibre-matrix interface and influence of the fabrication method on the interfacial bond will be discussed.

Through the process of extrusion the fibre packing and matrix density is increased, resulting in a stronger bond between the fibres and the matrix. This can cause the failure mechanism of the extruded SHCC composite to change from fibre pullout to fibre breakage, which will result in a more brittle type of failure. The addition of FA reduces the bond between the fibres and the matrix [Shah and Peled 2003]. This reduction in bond enhances the ductile behaviour of extruded SHCC by changing the failure mechanism form fibre breakage to fibre pullout.

The extrusion process also serves to enhance the density of the SHCC composite and thus reducing the porosity thereof [de Koker 2004]. It is widely acknowledged that the reduction of porosity leads to increased matrix strength and an increased E-modulus.

2.3.4 APPLICATIONS FOR EXTRUDED SHCC

There are numerous advantages associated with extrusion and the extrusion of SHCC. These advantages range from structural and mechanical advantages, superior to ordinary concrete, to better quality control. Extrusion has all the benefits associated with prefabrication, like better quality control, high geometrical tolerances and better accuracy but with an even higher production rate. It is also possible to create elements with complex geometrical cross sections through extrusion. The superior tensile mechanical characteristics of extruded SHCC in comparison to concrete and in many ways to cast SHCC make it a desirable material for structural use.

One specific application field for extruded SHCC is in permanent formwork. Extruded SHCC panels have been used for permanent formwork in the construction of bridges in Japan [Yamada et al. 2006]. Here, flat and curved panels with ribs were used as permanent formwork for the floor decks of bridges and the guard walls of the bridges. The use of cast SHCC as a permanent formwork has also been investigated at the University of Stellenbosch [Avenant 2005], where rectangular thin-walled, hollow tubes were cast from SHCC and used as permanent formwork for

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beams and columns. This research study showed that columns cast from ordinary reinforced concrete and columns cast from ordinary reinforced concrete, but in these prefabricated, hollow SHCC tubes and with 50% less shear reinforcement, are comparable based on axial load and bending moment resistance. The processes of casting and stripping the pre-fabricated SHCC tubes were complicated and remain to be improved if this method is to be used commercially. It is the opinion of the author that extrusion has good potential for manufacturing of such tubes.

The use of prefabricated concrete elements in combination with cast in-situ concrete is a concept called Hybrid Concrete Construction, a concept that is also currently being investigated at the University of Stellenbosch [Jurgens 2006] for the South African construction industry. Hybrid Concrete Construction can be defined as a method of construction which integrates prefabricated concrete and cast in-situ concrete to take best advantage of their different inherent qualities [Goodchild 2004]. It is the opinion of the author that the extrusion of SHCC can be utilized to good effect with this concept in mind. Extrusion can be used to produce uni-axial as well as possible bi-axial flexural members as well as other structural members like balustrades and facades.

Another field of interest for the use of SHCC is in the use of SHCC as a construction material in seismic active areas, due to its superior mechanical response to seismic loading in comparison with ordinary concrete [Fischer and Li 2002]. A lot of buildings that have been built in seismic active areas were not designed to resist seismic loading. One way of enhancing these buildings’ seismic resistance is by retrofitting structural members of such a building with SHCC. Extrusion can be used to manufacture SHCC panels that can be used for the epoxy-bonded retrofitting of R/C structural members [Farhat and Karihaloo 2007].

The enhanced performance of cast steel reinforced SHCC to that of ordinary reinforced concrete will be discussed in the next section along with the possible advantages of using extruded steel reinforced SHCC.

2.4 STEEL REINFORCED SHCC

2.4.1 CAST REINFORCED SHCC(R/SHCC)

The contribution of concrete to the load-deformation response beyond first cracking of reinforced concrete (R/C) in uni-axial tension is generally described as the tension-stiffening effect. The response of the R/C is compared with that of the bare steel reinforcement, and the difference is attributed to the tensile stresses in the concrete matrix between transverse cracks. Due to the

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brittleness of concrete, these cracks cannot transfer any significant stress across the crack and, consequently, the maximum tensile load of the reinforced composite is limited by the tensile strength of the reinforcement, assuming minimum reinforcement is provided. Besides this limitation on load-carrying capacity, cracking of concrete causes a deformation concentration in the steel that the surrounding concrete is unable to accommodate. In addition, high bond stresses are required to transfer the tensile load into the concrete matrix between transverse cracks. Both phenomena result in deterioration of the concrete matrix and adversely affect the intended composite mechanism. In order to overcome this inherent weakness, the concrete matrix has been substituted with various fibre-reinforced cement matrices in a number of research studies in which the effect of increased tensile strength and toughness by steel fibre reinforcement of concrete has been investigated. For example Abrishami and Mitchell [1997] reported that after cracking and significant deformation, plain concrete suffered splitting cracks and lost a significant amount of its stiffening contribution. The addition of steel fibres controlled these cracks and resulted in improved tension stiffening behaviour. Other studies similarly observed that high circumferential stresses lead to the development of longitudinal cracks, resulting in splitting and bursting of concrete cover [Krstulovic-Opara et al. 1994]. It has been found that the load level at which longitudinal cracking occurs is dependent on the cover thickness, the average bond stress at interface, and the composite tensile stress-strain response. Furthermore, an increase in bond strength and bond stiffness has been attributed to the presence of steel fibres in the concrete.

bare reinforcement R/C composite R/SHCC composite

Strain

Axial load

Figure 2.7: Typical load-deformation, tension-stiffening response of R/SHCC in comparison to

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Fischer and Li [2002] conducted an investigation into the influence of matrix ductility on the tension-stiffening behaviour of R/SHCC. An SHCC containing polyethylene fibres, with a tensile stress-strain response very similar to the one used in this research was used to emphasize the importance of the particular deformation characteristics of this SHCC. Uni-axial tensile tests were conducted on R/C and R/SHCC, where the SHCC have ductile deformation characteristics analogous to those of metals. An analysis of the deformation mechanisms suggested that the combination of steel reinforcement and SHCC results in composite action, where unlike R/C and FRC, both constituent materials deform compatibly in the postcracking and postyielding deformation process [Figure 2.7].

a) Composite before matrix cracking b) R/C after matrix cracking c) R/SHCC after matrix cracking

Figure 2.8: Schematic of crack formation and internal stresses in R/C and R/SHCC composites

[Fischer and Li 2002].

The comparison of R/C and R/SHCC showed qualitative and quantitative differences in their tension-stiffening behaviours. The combination of reinforcement and matrix material with elastic/plastic stress-strain behaviour results in a composite where both materials are deforming compatibly in the inelastic deformation regime. Consequently, damage induced by local slip and excessive interfacial bond stress between reinforcement and matrix is prevented. The SHCC matrix stiffens the specimen at uncracked sections and also strengthens it at cracked sections. Hence, the composite load-deformation response is significantly improved in terms of axial load-carrying capacity as well as ductility. To maintain this composite action at relatively large inelastic

Mechanical and structural characterisation of extrusion moulded SHCC